@misc{cryptoeprint:2010:225,
author = {Junghyun Nam},
title = {A Security Weakness in a Generic Construction of a Group Key Exchange Protocol},
howpublished = {Cryptology ePrint Archive, Report 2010/225},
year = {2010},
note = {\url{http://eprint.iacr.org/}},
}
You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.